1. Field of the Invention
The present invention relates to a manufacturing method of an epitaxial silicon wafer, and an epitaxial silicon wafer.
2. Description of Related Art
Epitaxial silicon wafers for power MOS transistors, for instance, is required to have extremely low substrate resistivity. In order to sufficiently lower the substrate resistivity of silicon wafers, it is known to dope molten silicon with an n-type dopant for resistivity adjustment (e.g. arsenic (As) and antimony (Sb)) during pull-up step (i.e. in growing silicon crystal) of a single crystal ingot for providing silicon wafers (referred to as a single crystal ingot hereinafter). However, since such dopants are extremely volatile, it is difficult to sufficiently increase the dopant concentration in the silicon crystals. Thus, silicon wafers exhibiting desired sufficiently low resistivity is difficult to be manufactured.
Accordingly, silicon wafers with extremely low substrate resistivity have come to be used in which phosphorus (P) as an n-type dopant that is less volatile than arsenic (As) and antimony (Sb) is doped at a high concentration (see, for instance, Literature 1: JP-A-2010-153631).
The Literature 1 discloses that, when an epitaxial film is grown on a silicon wafer provided by a single crystal ingot that is densely doped with phosphorus and germanium (Ge) while being grown, a number of stacking faults (abbreviated as “SF” hereinafter) are generated on the epitaxial film, the SF appearing on the surface of the silicon wafer in a form of steps to significantly deteriorate LPD (Light Point Defect) level on the surface of the silicon wafer.
In order to overcome the above deficiencies, the Literature 1 discloses that the epitaxial film is grown at a low temperature of 1000 to 1090 degrees C. with a CVD method after applying a prebaking treatment on the silicon wafer in a hydrogen gas atmosphere.
On the other hand, since epitaxial growth occurs on an epitaxial silicon wafer at a high temperature, oxygen precipitates (BMD) or oxygen precipitation nuclei formed in the crystal while growing the single crystal ingot are dissipated by the high temperature heat treatment, thereby lowering gettering ability.
In order to overcome the shortage in gettering ability, it is known to apply a polysilicon back-seal (PBS) before the epitaxial growth. The polysilicon back seal method is a kind of EG (External Gettering) in which a polysilicon film is formed on a backside of a silicon wafer to make use of strain fields or lattice mismatch created at an interface between the polysilicon film and the silicon wafer.
Literature 2 (JP-A-2011-9613) discloses that, in order to enhance the gettering ability, a polysilicon film is formed on a backside of a silicon wafer under a specific PBS condition before growing an epitaxial film on a silicon wafer provided by a single crystal ingot in which phosphorus and germanium are doped while growing the single crystal ingot.
Specifically, Literature 2 discloses that, since a large number of SF are formed on an epitaxial film even when a polysilicon film is formed on a backside of a silicon wafer, the SF appearing on the surface of the silicon wafer in a form of steps to significantly deteriorate the LPD level of the surface of the silicon wafer, the polysilicon film is formed on the backside of the silicon wafer at a temperature of less than 600 degrees C., whereby the creation of SF can be effectively restrained.
There is a recent need for an n-type silicon wafer of which substrate resistivity is 0.9 mΩ·cm or less. In order to respond to such a need, an epitaxial silicon wafer produced by forming an epitaxial film on a silicon wafer in which red phosphorus is densely doped when growing a single crystal ingot is required.
It is conceivable to apply the processes disclosed in Literatures 1 and 2 in order to manufacture such an epitaxial silicon wafer.
However, experiments performed by the inventors of the present application reveal that, when the substrate resistivity is extremely low (e.g. 0.9 mΩ·cm or less as in the above), generation of SF cannot be restrained even by applying the methods disclosed in Literatures 1 and 2, so that a high-quality epitaxial silicon wafer cannot be manufactured. In addition, it has been also revealed that, in accordance with the reduction in the substrate resistivity, red phosphorus emits out of a silicon wafer during the growth of an epitaxial film to be incorporated into the grown epitaxial film (auto-doping), which results in variations in the resistivity on the surface of the epitaxial film.